1. Field of the Invention
The present invention relates generally to a method and apparatus for the continuous manufacture of an insulated pipe, and more specifically, to a method and apparatus of manufacturing a linear wrap, thermally insulated tube.
2. Description of the Related Art
Pre-insulated tubes are frequently used in the process industry to transport steam, condensate and other hot or cold fluids. Tubes are also insulated and specifically designated as steam tracer lines for process piping. The operation of pumping a fluid through a process pipeline frequently requires that the process pipeline be heated to prevent the fluid from freezing, condensing or becoming too viscous to pump through the pipeline. Heating and maintaining the proper temperature of the process pipeline, which aids in the flow of the process fluids, is often accomplished by attaching a heat transfer tube or tracer line directly to the external surface of the process pipe. Often high temperature steam is available for use as the heating medium but will frequently deliver too much heat for low temperature or temperature sensitive applications. Hence an "isolated" tracer line or tube with insulation formed around it and having polyvinyl chloride (PVC), polyethylene or other moisture protecting polymeric jacket covering the insulation, is sometimes selected to retard the heat flow for such applications. Substantially accurate and consistent thermal conductivities of the insulating material used for both the tubes transporting hot or cold fluids and the "isolated" tracer tube is essential. The thermal conductivity or K value describes the rate at which heat is conducted through a specific material.
The pre-insulated transport lines must be energy efficient and provide a high degree of personnel safety by limiting the surface temperature of the pre-insulated lines to a temperature that reduces or prevents contact burn injuries.
The American Society for Testing Materials (ASTM) has developed Designation: C1055-92 titled "Standard Guide for Heated System Surface Conditions that Produce Contact Burn Injuries," and Designation: C1057-92 titled "Standard Practice for Determination of Skin Contact Temperature from Heated Surfaces using a Mathematical Model and Thermesthesiometer." ASTM C1055-92 establishes a process for the determination of an acceptable surface operating condition for heated systems, and states in summary that "Personal injury resulting from contact with heated surfaces can be prevented by proper design of insulation systems or other protective measures." ASTM C1057-92 establishes a procedure for evaluating the skin contact temperature of heated surfaces.
Typically, the insulating materials used for insulating pipes or tubes for the above-described purpose are fibrous materials, foamed materials, or other flexible insulants that can be shaped. These insulating materials have air- or gas-filled pockets, which will retard heat flow. The thermal conductivity of these insulating materials varies with density. Compressing these insulating materials increases density by reducing the air- or gas-filled spaces, which in turn increases the thermal conductivity. The increase in thermal conductivity increases heat loss and surface temperature.
The "isolated" tracer lines must have a consistent and uniform level of heat transfer between the tracer line and the process pipe so that predictable and reliable results are achieved to prevent over heating or under heating sensitive process lines.
A prior art method of continuously applying insulation to pipe or long coils of tube involved the helical winding of thin strips of thermal insulation material in an overlapping fashion. Multiple passes of helical wound insulation strips are applied and built-up to the desired thickness. The helical winding method of applying thermal insulation is a relatively slow process whereby large rolls of insulating material are typically rotated around the tube or pipe. Only insulating material of relatively high mechanical strength can be applied otherwise tearing of the insulating material will result. Generally, insulating material with high mechanical strength is denser resulting in increased thermal conductivity and lower thermal performance of the system. Inconsistency in the mechanical strength of the insulating material and the breaking force of the apparatus for applying the insulating material caused frequent tears in the insulating material resulting in substantial reduction in production efficiency. Inconsistency in the mechanical strength also caused variation in compression of the insulating material as it was applied to the tube or pipe resulting in variation of the finished outer diameter and variation in heat losses throughout the finished product. To arrive at a somewhat smooth surface, this method required the application of many thin layers of insulating material to minimize the height of the overlapping edges, otherwise a helical ridged spiral appeared on the finished surface of the product. Application of thin layers of insulating material aggravated the problem of tearing and slowed production efficiency.
U.S. Pat. No. 3,259,533 discloses a method and apparatus of advancing a strip of insulating material together with tubing in the longitudinal direction of the tubing through a stationary funnel-shaped die which compresses the strip of insulating material around the circumference of the tubing and captures the insulating strip as it exits the funnel-shaped die by helically winding a wire around the insulating material. The wire-wound insulated pipe can then be covered with a plastic coating for providing a moisture impervious pipe.
U.S. Pat. No. 3,594,246 discloses a method and apparatus of advancing a strip of insulating material together with tubing in the longitudinal direction of the tubing through a stationary funnel-shaped matrix which compresses the strip of insulating material around the circumference of the tubing and captures the insulating strip as it exits the funnel-shaped matrix by helically winding a tape around the insulating material.
U.S. Pat. No. 4,307,053 discloses a method and apparatus for guiding a strip of compressible insulating material lengthwise through a folding shoe and wrapping the strip material around a pipe and compressing portions of the strip material to increase the density of the strip material at the edges of the strip.
The prior art apparatus and methods in U.S. Pat. Nos. 3,259,533 and 3,594,246 discussed above describe continuously applying thermal insulation to tubes and pipes in a linear application of long strips of insulation material through a funnel-shaped compressing die. The insulating material is formed across its narrow axis into a cylindrical shape around the tube or pipe. Linear application of the insulating material eliminates forces that tear helical wound insulating materials resulting in significantly increased production efficiency. Production line speed is increased because of the elimination of rotating heavy rolls of insulating material and associated apparatus around the tube. Production line speed is also increased because thicker layers of insulating material may be applied without concern for creating helical ridges on the finished surface of the product. In the attempt to produce a product that has lower thermal conductivity, lower density insulating materials may be applied using this method. However, the advantage of applying lower density insulating material is offset in the prior art methods because of the compression created by the funnel-shaped compressing die. In the '533 and '246 patents, the insulating strips are compressed around the circumference of the tube or pipe. As the insulating strip and tube or pipe passes through the compressing die the oversized insulating strip is folded and compressed around the tube and immediately captured as it leaves the die and before compression is released by encircling with a tape-like material. The tape-like material encircles both the edge of the tip of the compressing die and the exiting insulating strip preventing the radial expansion of the insulating strip and maintaining the compressive forces. Compression of the insulating material causes an increase in density of the insulating material resulting in a decrease in thermal conductivity and thermal performance. The compressive forces are not distributed equally and vary continuously around the radial axis of the insulating strip causing inconsistencies in heat loss and thermal performance. With this method, it is also very difficult to maintain continuous contact at the seam where the two longitudinal edges of the insulating strip meet.
It is desirable to be able to apply a continuous longitudinal strip of compressible insulating material to a pipe or tube in a manner whereby compression of the insulating material is substantially eliminated. This would provide the advantage of allowing the insulating material to retain its thermal conductivity or K value after application to the pipe or tube which would result in the use of a reduced thickness or fewer layers of the insulating strip and allow for predictable modelling of insulated tube surface temperatures and tracer conductances. It is also desirable that this be accomplished without any sacrifice in production efficiency.